Capacitive sensing dryer control

ABSTRACT

A dryer control including capacitive sensing means which makes contact with fabric within the dryer. The resistivity of the fabric is a function of the moisture retained therein and determines the level of alternating current which flows through the capacitive sensing means. Current is monitored by a control circuit including a multistate transistorized amplifier which serves to disable an SCR which is connected in series with a timing mechanism. As the fabrics dry and become less conductive, less alternating current is transmitted by the capacitive sensor, de-energizing the control circuit and enabling the SCR, which allows current to pass through the timing mechanism.

United States Patent [1 1 Heidtmann CAPACITIVE SENSING DRYER CONTROL[75] Inventor: Donald S. Heidtmann, Louisville,

[73] Assignee: General Electric Company,

Louisville, Ky.

22 Filed: Sept. 22, 1971 21 Appl.No.: 182,580

[ Get. 16, 1973 Primary ExaminerMeyer Perlin Assistant Examiner-RonaldC. Capossela Attorney-Walter E. Rule et a1.

[5 7] ABSTRACT A dryer control including capacitive sensing means whichmakes contact with fabric within the dryer. The resistivity of thefabric is a function of the moisture retained therein and determines thelevel of alternating current which flows through the capacitive sensingmeans. Current is monitored by a control circuit including a multistatetransistorized amplifier which serves to disable an SCR which isconnected in series with a timing mechanism. As the fabrics dryandbecome less conductive, less alternating current is transmitted bythe capacitive sensor, de-energizing the control circuit and enablingthe SCR, which allows current to pass through the timing mechanism.

6 Claims, 2 Drawing Figures [52] U.S. Cl 34/45, 34/48, 73/3365 [51] Int.Cl. F26h 13/10 [58] Field of Search..; 236/44 E; 34/45, 34/48; 73/73,336.5

[56] References Cited UNITED STATES PATENTS 3,222,798 12/1965 Thornberryet a1 34/45 2,904,765 9/1959 Seehof et a1. 73/336.5 X 3,192,642 7/1965Hughes 34/45 3,411,219 11/1968 Bartholomew 34/45 2,930,016 3/1960 Westonet a1.. 73/336.5 X 2,962,897 12/1960 Muller.... 73/336.5 3,083,5734/1963 Shaw 73/336.5

PAIENTEU um I a ma INVENTOR. DONALD s. HEiDTMANN H\S ATTORNEY 1CAPACITIVE SENSING DRYER CONTROL BACKGROUND OF THE INVENTION The presentinvention relates to drying machines and, more particularly, to suchmachines having sensors placed therein for determining the moisturecontent of fabrics being dried.

In order to facilitate the operation of modern clothes dryers, suchmachines are commonly provided with a control for terminating theoperation of the machine when the moisture contentof fabrics therein hasbeen reduced to a predetermined level. In order to facilitate theoperation of the control, it is necessary to provide sensing means fordetermining the moisture content of the fabric. One popular approach tothis problem is to dispose a pair of sensing elements within the dryersuch that they are directly contacted by the fabrics. The fabrics thusact to bridge the gap between the sensor elements, completing anelectrical circuit and enabling the transmission of currenttherethrough. As the moisture content decreases the resistivity of thefabrics, and thus the resistivity of the electrical circuit portionconstituted thereby, is increased. .This change in circuit parameterscauses the control to provide an output signal whose level differs fromthat which was provided when the fabrics were moist, the new outputsignal being utilized to energize a timer which eventually terminatesthe operation of the machine.

SUMMARY OF THE INVENTION It is thus an object of the present inventionto=provide new capacitive sensing means in a fabric drying machine.

It is another object of the present invention to provide a sensor in afabric drying machine which does not make conductive contact with thefabrics therein.

A still further object of the present invention is to provide means forsimultaneously sensing the moisture content of fabrics that do notdirectly contact the sensor cover.

In accordance with one aspect of this invention, there is provided acontrol for a dryer which includes a timer for terminating the operationof the dryer after some predetermined time has elapsed, a control forenergizing the timer, and a capacitive sensing means for actuating thecontrol. A preferred embodiment of the sensing means includes a housingor'dome of temperatureresisting dielectric material which protrudes intothe chamber of a dryer containing fabrics to be dried. A

conductive coating disposed upon the inner surface of the housingconstitutes one plate of a capacitor, the insulated housing comprisingthe capacitor dielectric. The conductive coating is resistively. coupledto a source of alternating potential, and the enclosure within which thefabrics are disposed coupled to neutral, or ground potential. When thefabrics are moist, they act to electrically couple the groundedenclosure within the outer surface of the insulated sensor housing, thusforming what may be considered to be the second "plate of the capacitor.A flow of alternating current to the conducting material inside theinsulator housing is thus facilitated, and the voltage across theresistivecoupling resulting from the increased current flow iscommunicated to the control means.

In order not to affect the voltage drop across the resistive coupling,the initial stage of the control means comprises an amplifier having anextremely high input impedance. The amplifier serves to enable a finalstage of the control which disables an SCR connected in seriesrelationship with a timer motor. As the fabrics lose moisture currentflow through the resistive coupling decreases, de-energizing theamplifier and facilitating the conduction of the SCR, thus energizingthe terminating mechanism'or timer.

It will be seen that the capacitive sensing system which comprises thepresent invention has advantages not found in the prior art. Inparticular, the conductive path which couples the sensor to the groundedenclosure includes a substantial portion of the volume of fabriccontained within the enclosure rather than merely a small segment offabric which bridges a pair of closely-located electrical contacts as isthe case with prior art devices. A more representative sampling of theconductivity, and thus moisture content, of the fabric is thus achieved.

BRIEF DESCRIPTION OF THE DRAWINGS matter which is regarded as theinvention, it is believed that the invention will be better understoodfrom the following description of the preferred embodiment, taken inconnection with the accompanying drawings, in which:

FIG. 1 is a side elevational view of a fabric drying machine suitablefor incorporation of the improved dryer control;

FIG. 2 is a schematic circuit diagram of one embodiment of the presentinvention.

DESCRIPTION OF TI-IE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis illustrated a fabric drying machine 10 of the domestic type whichincludes a'cabinet 11 provided with a front door 12 for access to theinterior of an enclosure within. The enclosure, or drum, 14 is rotatablymounted within the cabinet and provides a means for retaining fabrics tobe dried. An electric motor 16 is disposed generally adjacent to thedrum 14. A pulley 17 at one end of the motor shaftis rotatably coupledto drum 14 by means of a drive belt 18. It will be understood by thoseskilled in the art that the speed or rotation of'drum 14 is determinedby the speed of motor 16, and the relative diameters of pulley l7 anddrum 14.

The rear end of the drum is supported within the cabinet upon a stubshaft 19 which is disposed along the axis of the drum, and is receivedin a stationary bearing means 20, affixed to the rear portion of thecabinet 11. The front end of the drum is rotatably supported upon a pairof rollers, or wheels, one of which is illustrated at 21. The wheels arerotatably mounted upon stub shafts, or axles, which are affixed to afront portion of cabinet 11, substantially parallel to the axis ofrotation of drum 14. The drum is thus rotatably supported at one end byan axle shaft and at the other end by rollers, so that it may be rotatedby motor 16 for causing fabrics contained therein to be agitated ortumbled in order to enhance the drying process. A plurality of clothestumbling ribs 31 are provided upon the lateral wall of drum 14 to enablethe fabrics to follow the lateral wall of the drum as it rotatesupwardly, tumbling back down as the rib ascends further.

A flow of air for drying the fabrics within the drum is drawn fromsuitable air inlets, such as louvers 22 at the rear of the cabinet, andpassed across electrical heating elements 23 and 24 after which theheated air traverses openings 25 in the rear end of the drum and passesaxially through the drum, and through the fabrics being agitatedtherein. The air leaves the fabric enclosing area by means ofperforations 26 in an inner bulkhead of door 12 and passes, by means ofaligned openings 27 and 28, into a duct 29 disposed within the frontwalls of the cabinet. A blower 30, rotatably driven by motor 16, impelsthe air drawn from duct 29 through an exhaust duct and thence from themachine.

A sensor 32 as taught by the present invention is mounted upon the innerbulkhead of door 12. The sensor housing advantageously has a dome-likeconfiguration, which will not snag or catch on fabrics as they passacross the outer surface thereof. The housing is preferably made of aheat-resistant plastic material such as Lexan, but may be made from anystrong heat-resistant dielectric material.

Referring now to FIG. 2, a schematic circuit is shown which comprehendsthe sensor and associated control and time delay means which terminateoperation of the dryer when moisture retained by the fabrics therein hasdeclined to some predetermined level. Terminals 40 and 41 are connectedacross a source of AC power which may, for instance, be 115 volt ACpower as is normally available in a household. In this case, terminal 41will represent neutral, or ground, potential. It will be recognized bythose skilled in the art that in cases where 230 volt AC power isprovided by means of a typical three-wire system, terminal 40 may beconnected to one side of the 3-wire supply and terminal 41 to theneutral conductor. Coupled between buses L, and N, connected toterminals 40 and 41 respectively, is a timing means. or motor 44 whichacts in a manner to be described hereinafter. Also coupled between thebuses, and thus across the AC power source, is a control which includestransistors 0,, Q Q and is supplied with rectified AC power by means ofadiode CR,. Sensor 32, includes, as schematically shown in FIG. 2, aninsulated housing 45 and a conductive element 43 which is coupled bymeans of current limiting resistor 46 and sensing resistor 47 to bus L,.The insulated housing 45 preferably comprises a dome-shaped element madeof a heat-resistant dielectrical material and the sensor is supported bya bulkhead so that the housing forms the inner surface of door 12.Disposed upon the inside of housing 45 is a conductive coating 43.Disposed upon the inside of housing 45 is a conductive coating 43. Thiscoating may be a metallic paint or other metallic coating which has beendeposited by a suitable process, such as vapor deposition. A conductoris electrically connected to the coating, and coupled by means ofresistors 46 and 47 to bus L,. Resistor 47 functions as a droppingresistor, providing a voltage which is representative of the currentflowing through the resistor to or from the conductive coating 43 of thesensor element. Resistor 46, of a much higher value than the resistor47, serves to limit current in the event that the sensor is damaged orshort-circuited. Cable shielding means indicated at 48 is provided aboutthe conductor which contacts resistor 46 to coating 43 and serves toshield the conductor from stray capacitance. To avoid undesirably highcurrent flow in the event of damage to the sensor, the cable shield iscoupled by means of current limiting resistor 42 to line voltage bus L,.A metal plate 33, interposed between the inner bulkhead of door 12 andsensing means 32, further shields the sensing means from straycapacitance which may be derived from the bulkhead.

Turning now to the control, a predetermined lower level of AC voltage isprovided by means of a voltage divider 50, including resistors 56 and57, to the anode of diode CR, which operates with filter capacitor C, toprovide filtered DC voltage to the control. Capacitor C integrates thevoltage appearing across resistor 51 in a manner to be described below.A first, high-gain amplifier Q, is provided and advantageously takes theform of a pair of transistors coupled in a Darlington configuration. Theemitter-collector circuit of the final, or second transistor of theDarlington circuit is resistively coupled between bus L, and resistor51, while the base of the first transistor of the pair, comprising theinput terminal of the amplifier Q, is coupled by means of conductor 52to the intersection of the dropping resistor 47 and current limitingresistor 46, for detecting the signal developed by the sensor 32.

The gain of amplifier Q, is extremely large, being the product of thegains of the two transistors comprising Darlington circuit. A smallchange in current, and thus voltage drop, across resistor 47 will thusresult in a significant change in the conductivity of the finaltransistor of the amplifier, and so of the voltage drop across resistor51. It will be recognized by those skilled in the art that various formsof amplifiers may be substituted for the Darlington configuration shownin the preferred embodiment. The amplifier selected, however, must havea high gain so that current drawn by the amplifier will not besufficient to disturb the voltage drop caused by current flowing throughresistor 47 to the capacitive sensing means. It will thus be understoodthat integrated circuits, FETs, and other high input impedance,high-gain amplification devices may be utilized, with suitable changesin the associated control circuit.

A diode CR is provided between conductor 52 and line voltage bus L, toprevent wrong polarity voltages from damaging amplifier Q,. A secondvoltage divider 53 including resistors 58 and 59 provides apredetermined level of DC voltage at the eimtter terminal of the finaltransistor of amplifier 0,, which determines the base voltage necessaryto drive the final stage of 0, into conduction. A further transistor Q,is coupled to the collector of the output stage of 0,, and the baseterminal ofa final transistor Q, is connected to the collector of QBiasing resistor 60 determines the input impedance of transistor 0,.Resistor 61 preventssmall leakage currents (through transistor Q frombeing conducted into the base terminal of transistor Q The collector oftransistor Q, is coupled to the positive side of the filtered DC voltageacross capacitor C, by means of a resistor 54.

Connected in series between timer motor 44 and line voltage bus L, isthe parallel combination of SCR,, diode CR and capacitor C As will beunderstood by those skilled in the art, diode CR conducts current in adirection opposite to that of SCR, and thus provides unidirectionalcurrent flow through timer motor 44 on alternating half cycles. In thepreferred embodiment the timer motor may be of the induction type andwill not operate on the half-wave current provided by CR, so that motoroperation commences only when SCR, is rendered conductive. The gateterminal of SCR, is connected directly to the collector of transistorQ3, and thus coupled by means of resistor 54 to the positive side of thefiltered DC voltage across capacitorC When the fabric drying machine isto be operated, a suitable AC voltage is impressed across terminals 40and 41, energizing the control circuit and sensor. Only half-wave AC isimpressed across timer motor 44 since SCR, is assumed not to be in theconductive mode.

In order to illustrate the operation of the capacitive sensing device,it will first be assumed that damp fabrics are disposed within the dryerdrum, and the control is energized. As will be recognized by thoseskilled in the art, the conductivity of the fabrics is directly relatedto the amount of moisture retained therein. The damp fabrics may thus bethought to comprise a conductive element, indicated by dotted lines at55 in FIG. 2. Conductive coating 43 and insulating housing 45 may beconsidered to be the plate and dielectric, respectively, of a capacitorcomprising the sensor 32. In order for the capacitor to conductalternating current there must necessarily be a second conductor orplate disposed upon the outside of the dielectric 45, and means forcoupling it to a source of neutral or ground potential. The damp fabricsconstitute thisconductive element, the electrical connection providedthereby being a function of the degree of moisture retained by thefabrics. As the drum 14 rotates, the fabrics are tumbled about, passingnear or against the outer surface of insulator 45 and forming acapacitor. The fabrics conduct the AC current from this capacitor to thesurface of the drum, which is electrically connected to source ofneutral or ground potential schematically represented by bus N. It willbe recognized that drums provided in fabric drying machines are oftencoated with enamel, porcelain, or other insulating substances. However,the thinness of the coating, combined with the relatively large surfacearea, merely has the effect of introducing a very large capacitance inseries between the damp fabrics and the body of the grounded drum. Theimpedance of this large capacitance to alternating current is very smallwith respect to other circuit parameters. Current is thus coupled by thedamp fabrics from the insulated housing of the capacitive sensor toground, facilitating current flow through dropping resistor 47. Thevoltage drop occurring across resistor 47 on alternate half cycles ofthe supply voltage will now be sufficient to bias amplifier 0. intoconduction.

When Q conducts the current drawn through resistor 51 causes a voltagedrop which tends to bias transistor Q into conduction. Capacitor Chowever, serves to integrate this voltage in order to prevent Q frombeing rendered conductive by occasional, spurious conduction ofamplifier Q Similarly, when a suitable voltage has been present longenough to bias Q into conduction, occasional lapses of conduction of Qon negative-half-cycles,due to intermitten conduction by isolatedsegments of moisture fabric, will not deplete the bias on Q The voltagelevel at which conduction of the amplifier takes place may be determinedby the relative size of the resistors in voltage divider 53, whichdetermines the DC voltage level at the emitter terminal of the finaltransistor of amplifier Q On half cycles when line voltage on bus L ispositive with respect to neutral bus N, current will flow through timermotor 44 only by means of diode CR bypassing SCR However, ashereinbefore noted, in the preferred embodiment timer motor 44 is of atype which cannot be energized by mere half-wave AC. When bus L becomesnegative with respect to neutral bus N, however, charge flows fromconductive coating 43 thr'ough limiting resistor 46, and thence throughdropping resistor 47. The intersection of the two resistors 46, 47 towhich conductor 52 isconnected then becomes sufficiently positive withrespect to bus L to energize amplifier 0,. Direct current is passedthrough diode CR and traverses resistor 51, biasing the base oftransistor Q such that Q now conducts. The voltage level assumed by thecollector of Q now becomes sufficiently positive with respect to bus Lto bias final transistor 0;, into conduction, shunting the gate terminalof SCR, to the cathode thereof and disabling SCR, from conducting. Itwill thus be seen that so long as the fabrics within drum 14 retainenough moisture to conduct current to the dielectric housing 45 of thecapacitive sensor the input terminal of amplifier 0 will be drivenpositive during negative half-cycles, disabling SCR, and preventing thetimer motor 44 from operating and terminating the operation of thedrying machine.

When sufficient moisture has been removed from the fabrics to renderthem relatively nonconductive, current flow to plate 43 of thecapacitive sensor, and thus current flow through dropping resistor 47,becomes insufficient to bias amplifier Q into the conductive state onnegative half-cycles. With amplifier Q non-conductive, insufficientcurrent will flow through resistor 51 to bias transistor Q intoconduction, thus precluding transistor Q from conducting and disablingSCR,. The resistivepath comprising resistor 57 of voltage divider 50,diode CR and resistor 54 thus provides a coupling from the gate terminalof SCR to apoint of neutral potential, energizing the SCR and allowingbidirectional current to flow through timing motor 44. Under theseconditions, SCR, will continue to conduct alternately with diode CRpassing full-wave alternating current through timing motor 44 andcontinuing the energization of the timing motor until the timingmechanism operated thereby terminates operation of the dryer.

I have found that a capacitive sensor and control circuit as illustratedin FIG. 2 for use with fabric dryers of the type described mayadvantageously be constructed with components having the followingvalues:

Components Values Resistor 42 I00 K ohms Resistor 46 470 K ohms Resistor47 2.2 M ohms Resistor 5] 270 K ohms Resistor 54 47 K ohms Resistor 563.3 K ohms Resistor 57 15 K ohms Resistor 58 4.7 K ohms Resistor 59 I5 Kohms Resistor 60 22 K ohms Resistor 61 47 K ohms SCR Cl06B (GeneralElectric) CR CR FDH-6l5 (Fairchild) CR 1N-5059 (General Electric) C 10 FQ Dl3P4 (General Electric Eleflrir) D29A5 (General Electric) Q 2N5l72(General Electric) It will be seen that it is possible to delete thetimer motor from the circuit, using a switched element such as the SCRprovided in the illustrated embodiment to energize other types ofmechanisms for terminating the operation of the dryer. It is alsopossible to terminate the operation of the machine immediately uponenergization of the SCR by switching means such as a relay. It willfurther be evident to those skilled in the art that other changes may bemade in certain aspects of the invention without departing from thespirit of the present invention. It is therefore intended to include bythe appended claims such modifications and variations as do not departfrom the true spirit and scope of the invention.

What is claimed is:

1. In a fabric drying machine having an enclosure for receiving moistfabrics to be dried:

a. capacitive sensing means comprising:

i. insulating means having a first surface exposed to the interior ofsaid enclosure for contacting said fabrics, and a second surfaceunexposed to the interior of said enclosure;

ii. first conductor means disposed upon said second surface of saidinsulating means to provide a first capacitor plate;

iii. second conductor means for coupling said first conductor means toone side ofa source of alternating potential;

iv. third conductor means for coupling said enclosure to a second sideof said source of alternating potential, said moist fabrics when tumbledwithin said enclosure providing an electrical path between saidenclosure and said insulating means to form a second capacitor platewith the conductivity of said path reflecting the degree of moistureretained by said fabrics; and

b. control means responsive to the level of current flow in said secondconductor means for terminating the operation of said fabric dryingmachine.

2. In a fabric drying machine having an enclosure for receiving moistfabrics to be dried, said enclosure having an insulating materialdisposed on the inner surface thereof:

a. capacitive sensing means comprising;

i. a dielectric housing having an outer surface exposed to the interiorof said enclosure for contacting said fabrics,

ii. conductive material disposed over the inner surface thereof toprovide a first capacitor plate,

iii. means for resistively coupling said conductive material to a sourceof alternating potential,

iv. means for coupling said enclosure to a point of ground potential,said fabrics when tumbled within said enclosure providing anelectricallyconductive path between said enclosure and said dielectrichousing to form a second capacitor plate with the conductivity of saidpath reflecting the degree of moisture retained by said fabrics,

b. control means for sensing the voltage across said resistive couplingmeans and providing an output representative thereof; and

c. timer means coupled to said control means for terminating theoperation of said fabric drying machine upon receiving said output fromsaid control means.

3. The invention as recited by claim 2, wherein said control meansincludes an amplifier means having a high input impedance and a highgain, said resistive coupling means being coupled to the input of saidamplifier, and said control means further including gated switchingmeans, said switching means being adapted to be operated in response toan output signal derived from said amplifier means.

4. The invention as recited in claim 2, wherein said control meanscomprises amplifier means having a high input impedance and including atleast one transistor, said resistive coupling means being connected tothe input terminal of said amplifier, said transistor being operativelycoupled to an output terminal of said amplifier means for disabling saidswitching means in response to a signal outputted by said amplifiermeans.

5. In a fabric drying machine, a capacitive sensor, comprising:

a. a rotatable enclosure for tumbling moist fabrics confined therein,said enclosure being adapted to be coupled to a first side of a sourceof alternating potential;

b. insulator means having a first and a second side,

said first side of said insulator means communicating with the interiorof said enclosure for contacting said fabrics, said fabrics providing anelectrically conductive path between said enclosure and said first sideof said insulator means to form a capacitor plate; and

c. conductor means disposed upon said second side of said insulatormeans to provide a capacitor plate and coupled to a second meansconnecting said capacitive sensor in a control circuit for controllingthe operation of the drying machine in response the moisture content ofthe fabric.

6. In a fabric drying machine having an enclosure for receiving moistfabrics to be dried:

a. capacitive sensing means comprising:

i. first conductive means being adapted to be connected to a first sideofa source of alternating potential and providing a first capacitorplate,

ii. insulator means having a first and second side,

said second side of said insulator means comm unicating with theinterior of said enclosure and said first side disposed adjacent saidfirst conductive means,

iii. second conductive means adapted to be connected to a second side ofsaid source of alternating potential and communicating with saidenclosure, said moist fabrics when tumbled within said enclosureproviding an electrical path between said second conductive means andsaid insulator means to form a second capacitor plate with theconductivity of said path reflecting the degree of moisture retained bysaid fabrics; and

b. control means coupled to said capacitive sensing means forterminating the operation of said fabric drying machine in response to apredetermined change in the amount of current flowing through saidcapacitive sensing means.

1. In a fabric drying machine having an enclosure for receiving moistfabrics to be dried: a. capacitive sensing means comprising: i.insulating means having a first surface exposed to the interior of saidenclosure for contacting said fabrics, and a second surface unexposed tothe interior of said enclosure; ii. first conductor means disposed uponsaid second surface of said insulating means to provide a firstcApacitor plate; iii. second conductor means for coupling said firstconductor means to one side of a source of alternating potential; iv.third conductor means for coupling said enclosure to a second side ofsaid source of alternating potential, said moist fabrics when tumbledwithin said enclosure providing an electrical path between saidenclosure and said insulating means to form a second capacitor platewith the conductivity of said path reflecting the degree of moistureretained by said fabrics; and b. control means responsive to the levelof current flow in said second conductor means for terminating theoperation of said fabric drying machine.
 2. In a fabric drying machinehaving an enclosure for receiving moist fabrics to be dried, saidenclosure having an insulating material disposed on the inner surfacethereof: a. capacitive sensing means comprising; i. a dielectric housinghaving an outer surface exposed to the interior of said enclosure forcontacting said fabrics, ii. conductive material disposed over the innersurface thereof to provide a first capacitor plate, iii. means forresistively coupling said conductive material to a source of alternatingpotential, iv. means for coupling said enclosure to a point of groundpotential, said fabrics when tumbled within said enclosure providing anelectrically-conductive path between said enclosure and said dielectrichousing to form a second capacitor plate with the conductivity of saidpath reflecting the degree of moisture retained by said fabrics, b.control means for sensing the voltage across said resistive couplingmeans and providing an output representative thereof; and c. timer meanscoupled to said control means for terminating the operation of saidfabric drying machine upon receiving said output from said controlmeans.
 3. The invention as recited by claim 2, wherein said controlmeans includes an amplifier means having a high input impedance and ahigh gain, said resistive coupling means being coupled to the input ofsaid amplifier, and said control means further including gated switchingmeans, said switching means being adapted to be operated in response toan output signal derived from said amplifier means.
 4. The invention asrecited in claim 2, wherein said control means comprises amplifier meanshaving a high input impedance and including at least one transistor,said resistive coupling means being connected to the input terminal ofsaid amplifier, said transistor being operatively coupled to an outputterminal of said amplifier means for disabling said switching means inresponse to a signal outputted by said amplifier means.
 5. In a fabricdrying machine, a capacitive sensor, comprising: a. a rotatableenclosure for tumbling moist fabrics confined therein, said enclosurebeing adapted to be coupled to a first side of a source of alternatingpotential; b. insulator means having a first and a second side, saidfirst side of said insulator means communicating with the interior ofsaid enclosure for contacting said fabrics, said fabrics providing anelectrically conductive path between said enclosure and said first sideof said insulator means to form a capacitor plate; and c. conductormeans disposed upon said second side of said insulator means to providea capacitor plate and coupled to a second means connecting saidcapacitive sensor in a control circuit for controlling the operation ofthe drying machine in response the moisture content of the fabric.
 6. Ina fabric drying machine having an enclosure for receiving moist fabricsto be dried: a. capacitive sensing means comprising: i. first conductivemeans being adapted to be connected to a first side of a source ofalternating potential and providing a first capacitor plate, ii.insulator means having a first and second side, said second side of saidinsulator means communicating with the interior of said enclosure andsaid first side disposed adjaCent said first conductive means, iii.second conductive means adapted to be connected to a second side of saidsource of alternating potential and communicating with said enclosure,said moist fabrics when tumbled within said enclosure providing anelectrical path between said second conductive means and said insulatormeans to form a second capacitor plate with the conductivity of saidpath reflecting the degree of moisture retained by said fabrics; and b.control means coupled to said capacitive sensing means for terminatingthe operation of said fabric drying machine in response to apredetermined change in the amount of current flowing through saidcapacitive sensing means.